The present invention relates to a clamp device that engages gripping claws of a gripping member into an aperture in a workpiece and pulls the workpiece against a clamp main body so as to clamp it, and in particular relates to a clamp device that can strengthen the clamping force while ensuring its support function for supporting the gripping member.
A clamp device of the above described type (i.e. a so called hole clamp or expansion clamp) is employed in cases such as when performing machining on the upper surface of a workpiece or the upper surface of the outer peripheral portion of a workpiece, since it is not possible to employ a clamp device of the type that applies pressure with a pressure member on the end portion of the workpiece from above. With this clamp device, a seating surface is defined on the clamp main body for seating the workpiece that is to be the subject of clamping, and, when the workpiece is to be mounted, by loading it on the seating surface and supporting it thereby, by inserting into an aperture in the workpiece a gripping member that grips the inner circumferential surface of the aperture in the workpiece and a clamp rod that has a tapered shaft portion inserted into this gripping member, and by pulling the clamp rod towards the seating surface by the force of hydraulic pressure or of a compression spring of a clamp drive means, the gripping member is expanded radially by the tapered shaft portion and is engaged with the inner circumferential surface of the aperture, and then the workpiece is fixed to the seating surface by further pulling this gripping member towards the seating surface. Clamp devices of this type are described in Patent Documents #1 and #2.
With the clamp devices of Patent Documents #1 and #2 described above, as a support mechanism that supports the gripping member from below, there are provided with an annular support member that supports the lower end of the gripping member, and a compression spring member that biases this annular support member upwards. Here, if the support force caused by the compression spring is insufficient, then when the clamping operation starts the gripping member will not grip the inner circumferential surface of the aperture, and will slip downwards. For this reason, as the compression spring that generates the support force, it is necessary to employ a spring capable of generating a support force that is rather large.
In some cases, as a support mechanism that differs from the support mechanism described above, there are provided with an annular pressure reception member that supports the lower end of the gripping member, and a hydraulic chamber that applies hydraulic pressure to the lower end portion of an annular cylindrical portion of this annular pressure reception member.
Patent Document #1: Japanese Patent Publication 3,550,010;
Patent Document #2: German Patent Publication 4,020,981.
With the support mechanisms of the clamp devices of the above Patent Documents #1 and #2, since support force is generated by the compression spring and biases the gripping member in the unclamping direction both during the unclamped state and during the clamped state, accordingly the clamping force is reduced by just the amount of this support force. For this reason it is not possible to strengthen the clamping force up to the maximum clamping force which can be developed by the clamp drive means, or even almost up to this maximum clamping force.
If, as a support mechanism, there are provided with an annular pressure reception member that supports the lower end of the gripping member, and a hydraulic chamber that applies hydraulic pressure to the lower end portion of an annular cylindrical portion of the annular pressure reception member, then it is often the case that the clamping hydraulic chamber that drives the clamp rod also is employed for generating the hydraulic pressure for the annular pressure reception member. With this structure as well, since before the start of the clamping operation a hydraulic pressure (i.e. a support force) acts on the annular pressure member in the opposite direction to the clamping force, accordingly the clamping force is diminished by just the amount of this support force. For this reason, it is not possible to strengthen the clamping force up to the maximum clamping force that can be developed by the clamp drive means, or even up to almost this maximum clamping force.